Update README.md

Author: yebai

README.md

@@ -1,5 +1,7 @@
 # Libtask
 
+[![Stable](https://img.shields.io/badge/docs-stable-blue.svg)](https://turinglang.github.io/Libtask.jl/stable)
+[![Dev](https://img.shields.io/badge/docs-dev-blue.svg)](https://turinglang.github.io/Libtask.jl/dev)
 [![Libtask Testing](https://github.com/TuringLang/Libtask.jl/workflows/Libtask%20Testing/badge.svg)](https://github.com/TuringLang/Libtask.jl/actions?branch=main)